Polyoptimisation of the refill friction stir spot welding parameters applied in joining 7075-T6 Alclad aluminium alloy sheets used in aircraft components

Abstract

In the aerospace industry, the refill friction stir spot welding (RFSSW) method is increasingly used to join thin sheets, especially those made of 2xxx and 7xxx series aluminium alloys. This paper presents the results of spot welding lap joints of 7075-T6 aluminium alloy sheets. The load capacity of joints was determined by tensile/shear tests using a universal testing machine. The effect of tool plunge depth, tool rotational speed and duration of welding on joint load capacity and failure mode was investigated. The polyoptimisation of the values of welding process parameters was carried out to ensure the highest load capacity of the joint at a minimum variance of the results obtained. The selection of the optimal parameters of the RFSSW process was carried out using an adequate mathematical model obtained on the basis of Weierstrass’ theorem. It was found that tool rotational speed had the greatest impact on the load capacity of the joints. It was possible to increase the load capacity of the joint by increasing the duration of welding but only to a limited extent. The selection of optimal welding parameters requires a compromise solution, i.e. the selection of a tool rotational speed that ensured adequate plasticisation of the material with a welding duration that ensured that an appropriate joint microstructure was obtained and assured the required load capacity of the joint. The methodology of mathematical modelling of polyoptimisation presented can be useful in optimising similar joining processes.